Support individual setup of model and optimizer in Lite

[awaelchli]

What does this PR do?

This PR is providing new APIs for setting up model and optimizers to enable FSDP in #14967.
The design considerations were discussed here.

Before:

Model and optimizers had to be set up jointly.

model = ...
optimizer = Adam(model.parameters())
model, optimizer = self.setup(model, optimizer)

Now:

Jointly setting up model and optimizer is still possible.

model = ...
optimizer = Adam(model.parameters())
model, optimizer = self.setup(model, optimizer)

For some strategies, this is the only way possible (DeepSpeed).

However, there are strategies (FSDP) that require to set up the model first before the optimizer can be created:

model = ...
model = self.setup_model(model)

optimizer = Adam(model.parameters())
optimizer = self.setup_optimizers(optimizer)

This PR introduces these two new APIs setup_model and setup_optimizers and the error messaging for which strategies support which setup procedure.

Does your PR introduce any breaking changes? If yes, please list them.

No.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @justusschock @awaelchli @carmocca

[Borda]

1. The API looks good to me 🦦
2. why are you updating _notebooks head?
3. shall we add also docs update for this?

[awaelchli]

This PR is a preparation for #14898. I think it will make most sense to add the docs there for full context.